Lasers are used in a wide variety of optical sensors. The natural frequency of a laser depends on the lasing medium and the cavity optics. However, once these parameters are chosen, the emission frequency is fixed and frequency stability of the laser beam is generally very good. Thus, in a conventional Helium-Neon laser a common wavelength of operation is 632.8 nm, corresponding to a frequency of 4.7408×1014 Hz. In a typical application, a pair of beams is derived from a single laser (for example using a beam-splitter), and the frequency of one of these beams is shifted by a fixed amount, such as in the tens or hundreds of MHz, as an aid to signal processing in an optical sensor-based system.
In a typical optical sensor-based system, such as a laser velocimeter or laser vibrometer, one of the beams may be used as a local oscillator, and mixed with light from the other beam after scattering from some remote moving object. This interaction will produce a carrier or difference frequency equal to the magnitude of the frequency shift. If the two beams, the local oscillator beam and the scattered and frequency shifted signal beam, are allowed to simultaneously fall on a suitable optical transducer, such as a photodiode, then a signal current will be produced which oscillates at the difference frequency, thus providing a carrier. If, now, the remote surface moves, then the signal beam will experience a Doppler shift a magnitude of which is arranged to be small compared to the static frequency shift. The result is that the photodiode (or other transducer) current will vary in frequency, i.e. the carrier frequency will be frequency or phase modulated according to the velocity of the remote surface. Electronic processing of the photodiode current or other transducer current can be used to retrieve information about the motion of the remote surface. If the motion of the remote surface is very fast, then very high frequency shifts of the optical beam are needed in order to ensure that the condition is met where the Doppler shifts are always less than the static frequency shift applied. If this condition is not met, then significant ambiguity regarding the velocity of the remote surface will be introduced, and the signal processing will not be able to correct the situation, giving rise to erroneous results.
Often, the necessary frequency shift is provided using an acousto-optic modulator (AOM), also called a Bragg cell. The AOM is generally in the form of a small enclosure typically a few cms cube, containing a suitably chosen acousto-optic crystal onto which an acoustic transducer placed between two electrodes is affixed in order to generate a traveling acoustic wave inside the crystal. The transducer thickness is chosen to match the acoustic frequency to be generated. Conventional AOM-based frequency shifters employ an isotropic type of AO interaction, in which the polarization state of the light is not changed during the diffraction process and such that the refractive index experienced by the incident and diffracted beam are the same (ni=nd).
The laser beam is incident on the crystal at θi through one of an opposing pair of apertures in the protective enclosure and the frequency shifted diffracted beam (which in practical applications is deviated a small angle from the incident beam) exits through the other. Because the frequency shifting process is not 100% efficient, a small amount of the residual incident beam (unshifted “zero degree” beam) also exits the Bragg cell device through the second (output) aperture. A frequency shift in the diffracted output beam is introduced by the acoustic interaction. Specifically, the diffracted beam f in order m, where m= . . . −2, −1, 0, 1, 2, . . . is the order of diffraction, will be Doppler-shifted by an amount equal to the frequency of the sound wave F.f→f+mF By vibrating the AO material with a pure sinusoid and tilting the AOM so the light is reflected from the flat sound waves into the first (m=1) diffraction order, 90% or greater deflection efficiency can be achieved. Obtainable diffraction efficiencies for higher orders are much lower. Thus, practical applications, the frequency shift for the shifted beam used is +/− equal to the acoustic frequency (F).
There are well known problems relating to producing a large frequency shift (e.g. >100 MHz) using a conventional AOM. Such AOMs typically produce either a 40 MHz or 80 MHz frequency shift as standard, and are tunable over a total frequency range of approximately 30% of the designed center frequency. Thus a device having a nominal 80 MHz centre frequency will be tunable from ˜68 MHz to ˜92 MHz and it will usually be set to some fixed value in this range. For some applications, it is necessary to produce much larger frequency shifts, in the range of hundreds of MHz up to and beyond 1 GHz. At these frequencies, the attenuation of the acoustic beam in the AOM crystal may become very large, as the acoustic attenuation increases typically as the square of acoustic frequency (the shift frequency). Therefore, due to acoustic attenuation, special techniques and designs are generally necessary in order to be able to place the acoustic transducer very close to the path taken by the optical beam through the interaction crystal.
It is important for ease of alignment and subsequent stability of operation of the instrument, for the efficiency of production of the shifted beam within the AOM(s) to depend as weakly as possible on the angle of incidence (θi) of the laser beam on the crystal. In conventional AOMs, it is necessary for the laser beam to be incident at the so-called “Bragg angle” which is determined and fixed during the design stage of AOM manufacture. The angular tolerance with which the laser beam may deviate from this condition in the standard (40 or 80 MHz) devices is small if they are to continue working efficiently, generally of the order of tens of milli-radians, and this makes alignment difficult.
The issue of angular acceptance is a function of more than just mechanical tolerances. In a typical well-adjusted Helium Neon laser, for example, the angular divergence of the beam (half-angle, measured to 1/e2 point) is ˜0.5 milli-radian in air. FIG. 1 shows a simplified schematic of a conventional isotropic-type AOM 100 comprising crystal (e.g. quartz) 105 having bonded transducer 107 having effective dimensions length (L) and height (H) as shown, both being defined by the geometry of top electrode 109 as known in the art. An absorbing structure 108 is shown opposite the transducer 107. Typically, L of the crystal is 3 to 50 mm and is chosen to give the required bandwidth and efficiency. H depends on the type of application, and must exceed the laser beam diameter. The transducer thickness is chosen to match the acoustic frequency to be generated and is typically 1 to 100 μm. The incident optical beam enters clear aperture 112, which can be AR coated.
The angular acceptance of a conventional AOM is given in terms of the acoustic wavelength and the length L of the transducer top electrode.
                              Δ          ⁢                                          ⁢          θ                ≈                  Λ                      π            ⁢                                                  ⁢            L                                              (        1        )            When using a conventional AOM crystal, at a frequency of 300 MHz, for example, the acoustic wavelength will be of the order of 10 μm, and the L is generally made to be on the order of 10 mm. Thus, Δθ≈0.3 milli-radians, according to equation (1). This is of the same order as the angular divergence of the laser beam. This means that any divergence or beam misalignment in excess of this value will cause a noticeable drop in efficiency of the device. L=10 mm, H=1.5 mm is what might be expected if trying to make a frequency shifter for a laser beam operating at fshift=300 MHz. However, if the transducer L is shortened (e.g. make L<10 mm) in an attempt to increase the angular acceptance given by equation (1), another problem arises, specifically, the inability to maintain diffraction efficiency at the previous value. Also, a transducer L=10 mm and H=1.5 mm, represents an electrode area of 15 mm2, which is difficult to impedance match (to avoid power return losses) to 50 Ohms at 300 MHz, and shortening it will make it easier to match as this obviously reduces the area, and hence increases the radiation resistance. However, more RF power will be needed to drive the shorter device in order to maintain a given level of efficiency, also it is possible that the acoustic divergence from this shorter transducer will become excessively large, in which case the operation of the device will tend to move out of the “Bragg regime” and into the “Raman-Nath” regime of diffraction, which is known to be undesirable for a device of this type as multiple diffracted orders are produced, of which only a single one is useful, and thus efficiency is lowered.
More RF power can also cause excessive heating of the transducer, and the resulting temperature rise of the AO interaction crystal (which is in direct physical contact with the transducer), which can significantly alter its optical properties. This unwanted effect would include the formation of a refractive index gradient, which will tend to steer and distort the outgoing beam. If the objective is to launch the output beam into a single mode optical fiber (as is the case with most sensors described herein), these effects will adversely affect the efficiency with which light couples into the core of the output fiber. This will contribute to further reductions in overall efficiency of the AOM-based frequency shifting system.
What is needed is an AOM-based frequency shifter that provides a larger frequency shift than the frequency shift provided by currently available AOM-based frequency shifters. Such an AOM-based frequency shifter would also preferably be less sensitive to angular beam alignment as compared to currently available AOM-based frequency shifters.